1. Field of the Invention
The present invention relates to a method of preventing the initiation of intergranular corrosion along the weld area of a welded ferritic stainless steel article, and more particularly to a method of grinding the heat affected zone on the torch side surface of a welded ferritic stainless steel article at a controlled depth along the length of the weld.
2. Description of the Prior Art
In recent years a demand has arisen for stabilized ferritic stainless steel articles such as welded tubing because of their application in extreme corrosion environments. Earlier unstabilized steels such as AISI 405, 410S and 430, were employed but did not exhibit the corrosion resistance required for recent applications. It has been found that welded articles made of ferritic stainless steel, such as 18-2 and 26-1 stainless steel, tend to exhibit sensitization in a shallow layer of the heat affected zone on the torch side of a weld. As used herein sensitization refers to increased sensitivity to intergranular attack in a corrosive media because chromium is combined with carbon and nitrogen rather than uncombined in the material. Although the reason for the phenomenon is not known for sure, it is speculated that weld heat and any atmosphere or lubricants carried or drawn into the weld area provide carbon, nitrogen and a thermal cycle to form chromium compounds.
Sensitization may be a result of contamination at the joined edges of the base metal, excessive heat from the welding operation or dissolution of stabilizing carbides into the heat affected zone. Regardless of the cause, sensitization occurs as the welded article cools from the welding temperatures at or above the melting temperature of the article, probably because carbon and/or nitrogen combines with chromium and locally depletes areas adjacent to grain boundaries.
Welded articles such as ferritic stainless steel tubing must meet intergranular corrosion test requirements as a prerequisite to acceptance or sale and use. During testing and evaluation it has been found that intergranular corrosion is predominantly initiated along the torch side surface of the weld, such as at the outside diameter of welded tubing, particularly at the weld-base metal interface. Accordingly, an effective method is desired for increasing the resistance to intergranular corrosion, and ideally for preventing the initiation of such intergranular corrosion.
In accordance with the present invention, a controlled grinding operation performed on the torch side surface of the weld on a surface area that exceeds the extent of the weld, and at a uniform depth along the length of the weld prevents the initiation of intergranular corrosion. Grinding of welded articles, such as tubing, has been practiced in the past. However, such grinding operations have been performed for aesthetic reasons. For example, it is desirable to have the exposed weld bead cosmetically dressed to enhance the appearance of the tube. Also, ornamental welded pipe is ground around the entire periphery of the outside surface to obtain a generally uniform appearance. Such prior art decorative grinding is not performed on a surface area that necessarily exceeds the weld area, and does not require a substantially uniform depth of grind along the length of the weld. Thus, such practices of cosmetically manicuring a weld bead would not prevent the initiation of intergranular corrosion.
Accordingly, an economical and effective process is desired for assuring that intergranular corrosion is not initiated at the weld-base metal interface of a welded ferritic stainless steel article.